Known methods of manufacturing plastic lenses include the cast polymerization method, in which a polymerizable plastic lens starting material liquid is cast into a casting mold and polymerized within the casting mold. To mass produce the plastic lens by cast polymerization, the steps of mixing multiple polymerizable components in a mixer and sequentially casting a mixed liquid discharged from the mixer into casting molds that are conveyed to a casting position are continuously conducted. When the initial polymerization rate of the plastic lens starting material liquid is relatively slow, the premixed liquid of plastic starting material liquid containing polymerizing components can be stockpiled in a tank or the like, the mixed liquid cast into a casting mold, and the fully cast casting mold conveyed from the casting position, with discharge of the mixed liquid being halted until the next casting mold is conveyed to the casting position. However, with the high-viscosity plastic lens starting material liquids described in Japanese Unexamined Patent Publication (KOKAI) No. 2004-291606 or English language family member US2005/0200033A1, Japanese Unexamined Patent Publication (KOKAI) No. 2006-205710 or English language family member US2008/0018005A1, and Japanese Unexamined Patent Publication (KOKAI) No. 2006-231600, which are expressly incorporated herein by reference in their entirety, which have rapid initial polymerization rates, casting was required immediately after mixing in the mixer when the conventional casting method is employed. This is because once the discharge from the mixer has been halted, the mixed liquid remaining in the mixer during the halt continues to polymerize. Thus, immediately following recommencement of discharge, the mixed liquid that has become highly viscous within the mixer is cast into the casting mold, causing a problem in the form of optical defects in the lens obtained. Alternatively, progression of polymerization of the mixed liquid within the mixer makes it difficult to recommence discharge.
The above-described problems can be avoided by not halting discharge of the mixed liquid from the mixer after recommencing casting of the mixed liquid. However, the mixed liquid continues to flow out during the period when a casting mold that has been fully loaded with mixed liquid is conveyed away from the casting position and the next casting mold is conveyed in, the mixed liquid that has flowed out is not cast into the casting mold. As a result, the quantity of starting material that is cast into the casting mold, that is, the effective use level, becomes small.